This invention relates to controlled atmosphere treatment processes that produce emissions that are environmentally unfriendly, and in particular, to the capture and handling of these emissions.
There are many types of apparatus for the treatment or processing of articles under controlled atmospheric conditions, such as an inert atmosphere, or other special temperature or atmospheric conditions. One category of these types of systems is what may be referred to as an open system, where the materials to be processed are fed on a conveyor through an enclosure that contains one or more zones where particular atmospheric conditions are maintained. After the materials or articles are processed in these controlled atmospheric conditions, the conveyor transfers them out of the enclosure. The conveyor can be continuous. Often, the enclosure has open or semi-open inlet and outlet openings where the conveyor passes through. The inlet and outlet openings are of sufficiently small cross-sectional area, or are fitted with flexible or movable curtains or labyrinth seals, so that a slight positive pressure is maintained inside the apparatus to continuously or intermittently purge the process chamber inside the apparatus. The control zones also usually have exhaust stacks for the venting or removal of emissions or unwanted by-products or purge gases resulting from the processing of the materials or articles treated in the apparatus.
Controlled atmosphere treatment processes depend on maintaining a specific level of atmosphere purity in the enclosure (process chamber) for process quality. To do this, inlet gas flows are adjusted to provide sufficient purge rates within the enclosure and through the exhaust stacks. The inlet flow rate is established to exclude infiltration of ambient atmosphere (air, moisture) into the enclosure, and also to sweep out process by-products or emissions. There are conflicting demands for a sufficiently low inlet purge rate so as to maintain a uniform distribution of pure gas within the enclosure (and for cost minimization), and a sufficiently high exhaust rate to remove process emissions. Therefore, a delicate balance must be maintained between the make-up atmosphere being supplied to the apparatus and the exhaust flows being removed from the apparatus.
Examples of the type of process or apparatus under consideration include controlled or zoned atmosphere furnaces or ovens for the soldering of electronic components, for sintering powder metal components, or for the brazing of articles such as metal heat exchangers. An example of this type of furnace used for soldering is shown in U.S. Pat. No. 5,573,688 issued to Chanasyk et al. In this type of furnace, an inert atmosphere is used, such as nitrogen, and successive zones are provided in the furnace to heat the articles gradually until they are soldered and then cool them down before emerging from the oven. In sintering or brazing furnaces, especially for oxidation sensitive components such as aluminum heat exchangers, specific temperature profiles, product feed rates and oxygen concentrations inside the various zones must be precisely controlled to provide the necessary protective or reducing atmospheres required for brazing, and to produce high quality brazed or sintered products. For example, in brazing aluminum heat exchangers, oxygen concentrations must be maintained in the low parts per million range by metering the supply and exhaust rate of the nitrogen protective gas used in the furnace. The purge rate of the protective gas must be sufficient to remove residual water vapour entrained by the product load, and any residual press lubricants on the product surfaces that will be volatized as the product is heated in the process (referred to as thermal degreasing), along with other process materials or by-products. The exhausted purge gas may carry contaminates, metals and oils from the furnace or process enclosure through the exhaust stacks in the form of particulates, vapors and gases. This may also be referred to as thermal degreasing of the components to be processed in the furnace. In a typical brazing process, the emissions can contain metals such as aluminum, cadmium, chromium, lead, bismuth, tin, iron, copper, magnesium, nickel and zinc, and also volatile organic compounds, such as thermal decomposition products including benzene, ethylbenzene, toluene, xylenes and others, and brazing fluxes or process by-products. Under current environmental regulations, it may not be possible simply to exhaust these environmentally unfriendly emissions to the atmosphere. It may be necessary to collect them and treat or dispose of them properly.
One method that has been used to collect these environmentally unfriendly emissions in the past has been to place an exhaust hood over each exhaust stack and use an exhaust fan or other suction device to draw the exhaust stack flow into the exhaust hoods. However, the efficiency of this type of captured device is very poor. Sometimes, an ejector is used in the furnace exhaust stacks to help remove the emissions from the furnace and direct them to the exhaust hoods, but this does not help very much and it requires an extra flow of inert gas to operate the ejectors, since air cannot be used or the oxygen in the air may infiltrate the controlled zones through the exhaust stack. It has not been thought possible in this type of system to directly connect the furnace exhaust stacks to the exhaust hoods to ensure full capture of the exhaust emissions, because the flow rates are very low and the inert gas inlet and exhaust removal cannot be controlled accurately enough to maintain the desired controlled atmosphere inside the furnace without causing infiltration from outside the furnace.
The present invention is able to provide a directly coupled exhaust system by providing metering devices connected between the furnace exhaust stacks and a suction source and controlling the suction applied to the metering devices to maintain the inlet flow thereto generally equal to desired predetermined flow rates through the exhaust stacks. In so doing, a uniform microclimate of undisturbed atmosphere can be maintained within the process enclosure at a uniform positive pressure with respect to the ambient atmosphere.
According to one aspect of the invention, there is provided an exhaust system for a controlled atmosphere treatment apparatus having at least one internal zone with predetermined atmospheric conditions therein and an exhaust stack for a predetermined exhaust flow from the zone. The system comprises a metering device attached in fluid communication with the exhaust stack. The metering device has a converging entrance portion defining an inlet, a reduced diameter outlet portion defining an outlet, and a predetermined relationship between the inlet and outlet flow therethrough. Suction means is attached to communicate with the metering device outlet for drawing exhaust flow through the metering device. Also, suction control means is operably associated with the suction means for maintaining the outlet flow in the metering device such that the inlet flow thereto is generally equal to the predetermined exhaust flow from the internal zone.
According to another aspect of the invention, there is provided a method of capturing emissions from a controlled atmosphere treatment apparatus of the type having an internal zone with predetermined atmospheric conditions therein and an exhaust stack for a predetermined exhaust flow from the zone. The method comprises the steps of increasing the flow velocity of the exhaust flow from the exhaust stack while maintaining constant the mass flow rate through the exhaust stack. The increased velocity flow from the exhaust stack is delivered to a remote location. Also, the temperature and velocity of the exhaust flow is controlled during the delivery step so as to prevent condensation and precipitation of volatiles and particulate matter from the exhaust flow.